PHP reproduce warning "Cannot modify header information - headers already sent by (output started at... " with XAMPP 8..1.1

Question

I'm using XAMPP 8.1.1 as dev. environment. In the php.ini is set: error_reporting = E_ALL

With this code I'm expecting the warning: "Cannot modify header information - headers already sent by..."

<?php
echo 'It works!<br>';
//Expected Warning "Cannot modify header information - headers already sent by"
header ('Content-Type: text/html; charset=utf-8');

echo '<br>...not!';
?>

If I call this function my ISP, I will get the warning, but not on my XAMPP installation. What should I change, that I will get this warning too on my XAMPP installation? I don't have access to the php.ini File at my ISP to compare the settings, all I found out, is that the error reporting is a little bit less strict, but with the same settings on my XAMPP, I do even not get the warning.

Thank you very much for your help.

Answer 1

Your ISP has output buffering disabled. On your XAMPP installation it's enabled. Outputting content before sending headers will not generate the warning unless content length exceeds configured output_buffering value (4096 by default). https://www.php.net/manual/en/outcontrol.configuration.php